For recording on recording media such as papers, etc., by writing instruments (e.g., a fountain pen, a felt pen, etc.), there have hitherto been used inks, which are solutions of various dyes dissolved in water or organic solvents.
Also, a so-called ink-jet recording system wherein recording is performed by ejecting a recording liquid (droplets of a recording ink) in a recording head through discharging orifices by the vibration of piezo oscillators, the electrostatic attraction caused by the application of a high voltage, etc., and for the recording system, recording liquids obtained by dissolving various dyes in water or organic solvents are used.
Such a recording liquid is fundamentally composed of a recording agent (a dye or a pigment) and a liquid medium (water, an organic solvent or a mixture of them) for dissolving or dispersing the recording agent and, if necessary, various additives are added to the recording liquid.
For the ink-jet recording liquid, more restrict performance characteristics are requested than for a general writing ink for writing instruments such as a fountain pen, a felt pen, etc. Preferred conditions required for this kind of recording liquid are as follows:
(1) The recording liquid possesses liquid properties (viscosity, surface tension, electric conductivity, etc.) matching the discharging conditions (a driving voltage and a driving frequency of a piezo electric element, the form and material of orifices, the diameter of orifices, etc.),
(2) The recording liquid can be stored for a long period of time and does not cause clogging of orifices at use.
(3) The recording liquid is quickly fixed onto recording media such as papers, films, etc., the outlines of the resulting ink dots are smooth, and blotting of the dotted ink is minimized.
(4) The printed or recorded image by the recording liquid has a clear color tone and a high density.
(5) The printed or recorded image by the recording liquid is excellent in water resistance and light resistance.
(6) The recording liquid does not attach or erode surrounding materials (the container for the recording liquid, connecting tubes for discharging orifices, sealants, etc.).
(7) The recording liquid has no unpleasant smell, toxicity, inflammability, etc.
However, it is very difficult to obtain the recording liquid which satisfies all the foregoing conditions at the same time.
Also, since the recording liquid which is used for such a recording purpose is fundamentally composed of a dye and a solvent for the dye, the characteristics of the recording liquid greatly depend upon the property specific to the dye. Accordingly, it is very important in the field of the art to select a dye so that the recording liquid containing the dye has the foregoing characteristics.
As this kind of recording liquid, there are known solutions or dispersions of various dyes or pigments in aqueous or nonaqueous solvents as disclosed, for example, in Japanese Patent Publication Nos. 8361/'75; U.S. Pat. No. 3,846,141; and No. 3,705,043; Japanese Patent Publication No. 13,127/'77; U.S. Pat. No. 3,870,528, etc., and as black dyes for use in these recording liquids, following dyes (a) to (d) are known. ##STR2##
Also, as black naphthalenedisazoic dyes, the dyes having following structural formulae are known. ##STR3##